Photolithography or optical lithography is a process used, inter alia, in semiconductor device fabrication to transfer a pattern from a photomask (sometimes called reticle) to the surface of a substrate. At present, it is desired to use light in the extreme UV range (13.5 nm or shorter) for photolithography of circuits having line widths of 32-20 nm, for which it has been proposed to utilize “chemically amplified” photoresists. The idea is to include in the photoresist an amount of a thermally stable, photolytically activated acid precursor (sometimes called a “photoacid generator” or “PAG”), so that upon irradiation, acid will be generated. The polymer in the photoresist is chosen to be acid labile so that the acid from the PAG will cleave the polymer. In a variation, it has been proposed to include in the resist composition—in addition to the photoacid generator—an acid precursor (sometimes referred to as an “acid amplifier”) which is (a) photolytically stable and (b) thermally stable in the absence of acid but thermally active in the presence of acid. In such systems, during radiation exposure the PAG generates acid, which then during post-exposure bake acts as a catalyst to activate the acid-amplifier. Such systems are sometimes referred to in the literature as “acid amplifier” systems, since the catalytic action of the photolytically-generated acid on the second acid precursor during post-exposure bake results in an effective number of acid molecules which is higher than the number of photons absorbed during radiation exposure, thus effectively “amplifying” the effect of exposure and amplifying the amount of acid present.
Among the difficulties encountered in trying to implement chemical amplification photoresists systems is “outgassing”, a process whereby, as a result of acid formation, gas is generated, leading to volatile compounds that can leave the resist film while the wafer is still in the exposure tool. Outgassing can occur under ambient conditions or under vacuum as is used with extreme ultraviolet (EUV) lithography. Outgassing is a problem because the small molecules can deposit on the optics (lenses or mirrors) of the exposure tool and cause a diminution of performance. Furthermore, there is a trade-off between resolution, line-width roughness and sensitivity. A resist's resolution is typically characterized as the smallest feature the resist can print. Line width roughness is the statistical variation in the width of a line. Sensitivity is the dose of radiation required to print a specific feature on the resist, and is usually expressed in units of mJ/cm2.
PCT application PCT/US09/34707 filed Feb. 20, 2009 discloses a new resist system based on a polymer with PAG and ester functionality located within the main polymer chain. When the PAG breaks apart photochemically or the ester-linkages break apart by acidolysis, the molecular weight of the polymer decreases, allowing for higher acid diffusion during bake and faster resist dissolution during development. Acid-labile diols are useful monomers for incorporating in such polymers.